A post-mortem was being performed on the head at the WA State Mortuary in Nedlands Monday afternoon, when – 24 hours after the discovery – police confirmed the head was human.

"The plastic contained the remains of a male Caucasian head," Detective Western said.

"We believe he may have been aged between 30 and 50 years – though those ages aren't definitive."

The head had no piercings.

But there was one distinctive characteristic police hoped would help identify its owner – there were no upper front teeth.

The examination of the head revealed the teeth had been lost sometime before its owner met "whatever fate he's met," Detective Western said.

Porpoise Bay was declared a protected forensic area and closed off to the public as police conducted searches of the Island's southern coastline.

"The death is being treated as a homicide investigation," Detective Western said.

"We are conducting investigations to determine the currents surrounding that area.

"The avenues that things could have arrived from are limitless."

Police have not ruled out the possibility the head may have washed up on Rottnest from the mainland or somewhere else entirely.

"We don't know so the searches are really to make sure that we do search the entire southern face of that Island which is where most of the debris that washes onto that Island washes up," Detective Western said.

Boats located in Porpoise Bay at the time of the discovery have been identified and police plan to speak with their occupants in the coming days.

Police are appealing for anybody with information that may assist them to contact Crime Stoppers on 1800 333 000.